Application of micro- and the air permeability due to an increase in the bonded area within the fiber network that closes off the porous structure.5 nanofibrillated cellulose (MNFC) in The reduction in air permeability correlates with an increase hygiene tissue products in the sheet density.6 Addition of MNFC also reduces the drainage rate of pulp slurries. Proper selection of retention aids helps to reverse this trend to some extent.4 Franklin Zambrano, Richard Venditti, Hasan Applications of MNFC in papermaking have mainly Jameel, Ronalds Gonzalez* focused on printing and writing paper7 and, to a lesser degree, on packaging materials.8 For such grades, Department of Forest Biomaterials, North Carolina State mechanical stability represents a critical control parameter University, Raleigh, NC 27695-8005, USA. that can be enhanced by the addition of MNFC. In tissue *Corresponding author: [email protected] papers, strength properties are only necessary to meet stresses and strains requirements during manufacturing ABSTRACT operations and consumer usage.9 For this particular grade, other physical parameters highlight according to the The development of novel technologies and materials application for which each tissue product is designed. to face current and future market challenges related to fiber Softness, absorbency, and brightness are desirable for bath supply, quality and cost is a hot topic for the hygiene tissue and facial tissue, whereas absorbency and strength are more industry. Micro- and nanofibrillated cellulose (MNFC) important for napkins and kitchen towels.10 presents an opportunity to modify tissue paper features due The addition of MNFC can be regarded as a wet-end to its superior strength properties. This has triggered a strategy to control tissue properties. To the best of the growing interest in using MNFC to generate either high- authors’ knowledge, no comprehensive studies have been value applications or low-cost alternatives. This work aims published on applications of MNFC in light-weight papers to develop a systematic study on the use of MNFC in with a focus on hygiene tissue. In that regard, the objective hygiene products with a focus on tissue paper. To that end, of the present work is to study the effect of the addition of MNFCs produced from four different market pulps were MNFC on the major properties of hygiene tissue products. added to tissue-making slurries of virgin fibers at different To that end, the performance of four MNFCs prepared from loads. Results show that MNFC act as an effective strength virgin and recycled market pulps is simultaneously additive for tissue paper, however, there is a tradeoff evaluated at different loads in a tissue-making slurry. The between the strength gains and the changes in other tradeoff between the resulting tissue properties, namely properties such as bulk, water absorbency, and softness, tensile strength, softness and water absorbency, is analyzed which are negatively affected. The extent of the changes and the MNFC that performs best for each specific tissue depends on both the load and the type of MNFC. As strength application is recommended. The ultimate goal of this work requirements are necessary to withstand tissue- is to screen opportunities for value creation in the hygiene manufacturing operations, this allows tailoring the final tissue industry by using renewable nanotechnology. tissue properties according to the application for which each hygiene product is designed. EXPERIMENTAL Keywords: Nanofibrillated cellulose (NFC); Materials Microfibrillated cellulose (MFC); Hygiene tissue paper; Micro- and nanofibrillated celluloses prepared from four Tensile strength; Softness; Water absorbency different market pulps through mechanical fibrillation in an ultra-fine friction grinder Supermasscolloider (model INTRODUCTION MKZA6-5, Masuko Sangyo Co., Ltd, Saitama, Japan) were used as strength additive for tissue paper. The MNFCs The use of nanotechnology is an emerging area that is samples were taken once a total net energy input of 6,000 finding its way into daily consumer commodities.1 Within kWh/od ton was reached during fibrillation. Table 1 this segment, micro- and nanofibrillated cellulose (MNFC) describes the market pulps selected for MNFC is the potential candidate to create high-end products or low- manufacturing, including aspects related to fiber cost alternatives for the pulp and paper industry.2 This field morphology, and “as received” moisture content. represents a promising arena for bio-nanomaterials aiming Fig 1 shows SEM images of the MNFC produced from to achieve a short-term adoption by the market.3 each feedstock along with the average fibrils width obtained In papermaking applications, adding MNFC to pulp from an image analysis software (ImageJ 1.47v, National slurries yields paper products with improved mechanical Institutes of Health, Bethesda, MD). The average width of properties. Several authors have reported the strengthening the fibrils was calculated considering a minimum of 50 capacity of MNFC in paper products.4 In addition to randomly selected particles from at least five images per increasing the strength properties of paper, MNFC decreases feedstock. Pulp slurry preparation this context, the procedure followed for forming handsheets A tissue-making slurry consisting of 70 wt.% SBHK and for physical tests of pulp correspond to a modified version 30 wt.% NBSK was used to make 30 g/m2 light-weight of TAPPI T 205 sp-0212. This alternative method intends to paper. Two loads (1 wt.% and 2 wt.% based on od fiber) of adapt the standard procedure to the conditions used in tissue MNFC produced from the various feedstocks were added to manufacturing. Briefly, pressing and ring drying of the the slurry. MNFC and papermaking fibers were mixed handsheets are avoided. Instead, after formation and together at 3000 rpm during 5 min and 1.2 wt.% consistency couching, handsheets are dried using a cylindrical dryer using a pulp disintegrator (Testing Machines, Inc., (Formax 12”, Adirondack Machine Co., Gleans Fall, NY) Amityville, NY). After disintegration, the MNFC-pulp that simulates the yankee roll operation. The operating slurry was diluted to 0.3 wt.% consistency using tap water. conditions for the cylindrical dryer are 110°C, 20% of nominal speed, and 5 min residence time. Handsheets Table 1. Market pulps used for MNFC and tissue produced from this method are uncreped. handsheets preparation Pulp type Hardwood Softwood Recycled Handsheets testing Evaluation of handsheets mechanical and physical Southern Northern Bleached bleached bleached Deinked properties was performed according to ISO 12625. Pulp name Eucalyptus hardwood softwood pulp Handsheets samples were conditioned for 24 hours under a Kraft Kraft Kraft standard atmosphere set at 50% relative humidity and 23°C Pulp tag BEK SBHK NBSK DIP before testing (ISO 187).13 Softness, defined in this context Fiber length1 as the human sensorial response obtained when a tissue 0.802 1.101 2.343 1.111 (mm) product is stroked with fingers and crumpled by hand,15 was Mean width 15.6 17.9 25.7 17.9 assessed with a Tissue Softness Analyzer (Emtec Electronic (µm) Fines content2 GmbH, Leipzig, Germany). 31.35 73.67 56.25 55.73 (%) Moisture 7.72 7.87 6.41 7.34 RESULTS AND DISCUSSION content3 (%) 1Length weighted mean length; 2Arithmetic percent fines. Fines are defined as particles with length between 0.025 mm and 0.2 mm; 3 “As received” moisture content This section describes the changes in the physical and measured in accordance with TAPPI T 412 om-1611 mechanical properties that occur to the tissue paper due to Fiber morphology determined with fiber quality analyzer (HiRes FQA, OPTest Equipment Inc., Hawkesbury, ON, Canada) the addition of MNFC. For each property studied, results are discussed considering the effect of the load and type of MNFC used. Tensile strength of tissue paper The addition of MNFC to the tissue-making slurry enhances the tensile strength of the resulting tissue paper. The increase in the tensile strength promoted by the MNFC addition is shown in Fig 2. Overall, the increase in tensile strength is proportional to the MNFC load in the slurry. The a) BEK-MNFC b) SBHK-MNFC Average fibrils width = 284 nm Average fibrils width = 165 nm performance of the MNFC as a tissue strength additive also varies according to the feedstock used for MNFC manufacturing. At a 1 wt.% load, small differences exist between the types of MNFC. Interestingly, differences in strength improvement become more noticeable with the increase in the MNFC load. At a 2 wt.% load, MNFC produced from NBSK yields the stronger tissue structure with an increase of 67% in the tensile strength compared to the base case. Conversely, BEK-MNFC triggers an increase c) NBSK-MNFC d) DIP-MNFC Average fibrils width = 130 nm Average fibrils width = 213 nm of 30%, which is the lowest increase obtained among the conditions studied. Fig 1. SEM images for MNFCs prepared by mechanical The differences in performances showed a correlation fibrillation of (a) BEK, (b) SBHK, (c) NBSK, and (d) DIP with the average fibrils width for MNFCs prepared from virgin fibers. NBSK-MNFC, which has the smallest fibrils Handsheets making width (130 nm) presents the major improvement in tensile In tissue manufacturing operations, wet pressing of the strength. Contrarily, BEK-MNFC, which has the largest paper web is minimized to preserve bulk as densification fibrils width (284 nm), shows the poorest performance. yields poor softness and absorbency of the paper sheet. In SBHK-MNFC, with an intermediate average fibrils width (165 nm), shows a performance that is between the two preserve bulk, even though pressing is a cost-effective MNFCs mentioned before. The strengthening mechanism of technique to partially dewater the paper web and promote MNFC is based on the increase of the total bonded area by consolidation of the sheet. In practice, finding an optimum promoting fiber-fiber bonding, and the formation of ratio between the tensile strength and the bulk represents a embedded nano-networks along larger fibers that serve as challenge.